1. Field of the Invention
This invention relates to a spark plug which has a metallic shell extended into a combustion chamber of an internal combustion engine.
2. Description of Prior Art
In a spark plug which is usually employed in an internal combustion engine, it has been suggested that a firing tip is protracted into a combustion chamber of the internal combustion engine to improve ignition efficiency.
With the protraction of the firing tip, a front end of the metallic shell is extended into a combustion chamber to protect an outer electrode against excessive heat.
In order to prevent the outer electrode from being accidentally broken when the outer electrode is bent, a tapered surface is provided with a front end of the metallic shell to reduce a bending degree of the outer electrode when the outer electrode is welded to the tapered end surface of the metallic shell.
Further, it has been taught to decrease an annular clearance between a front end of the metallic shell and that of an insulator located within the metallic shell. This is a countermeasure against fouling of a front end of the insulator, an option of which is to remove an accumulation of carbon deposit on the insulator by spark discharge occurring through the annular clearance.
With a recent high-output performance of the internal combustion engine, it is required to prevent the outer electrode from being accidentally broken, and at the same time, protecting the insulator against fouling when operating the engine with a low load.
To cope with this requirement, it is considered to adopt the individual countermeasures in combination.
It, however, is found that only combining the individual countermeasures leads to interfering the individual advantages so as to bear no good results.
Therefore, it is an object of the invention to provide a spark plug which is capable of maintaining good ignition without misfire, and preventing an outer electrode from being accidentally broken, and at the same time, protecting the insulator against fouling when operating the engine with a low load.
Then, the invention contributes to an extended period of service life, and achieving these effects with a relatively simple construction.